Scalable Synthesis of Fmoc-Protected GalNAc-Threonine Amino Acid and T_N Antigen via Nickel Catalysis

Abstract

The highly α-selective and scalable synthesis of the Fmoc-protected GalNAc-threonine amino acid and TN antigen in gram scale (0.5 – 1 gram), is described. The challenging 1,2-cis-2-amino glycosidic bond is addressed through a coupling of threonine units with C(2)-N-ortho-(trifluoromethyl)benzylidenamino trihaloacetimidate donors mediated by Ni(4-F-PhCN)4(OTf)2. The desired 1,2-cis-2-amino glycoside was obtained in 66% (3.77 g) with α-only selectivity and subsequently transformed into the Fmoc-protected GalNAc-threonine and TN antigen. This operationally simple procedure no longer requires utilization of the commonly used C(2)-azido donors and overcomes many of the limitations associated with the synthesis of 1,2-cis linkage.

Utilization of bench-stable and readily available nickel(II) triflate for access to 1,2-cis-2-aminoglycosides

“…Synthesis of these donors requires the use of the TfN 3 and when not handled properly can be highly explosive. 21 Although we have illustrated that use of 15 mol % Ni(4-F-PhCN) 4 OTf 2 was effective at safely providing the threonine-containing T N antigen derivative 23 in high yield and α-anomeric selectivity (Scheme 4a), 6f the major drawback is to prepare Ni(4-F-PhCN) 4 OTf 2 in reproducible and large quantities. Thus, our goal is to investigate the ability of Ni(OTf) 2 to provide large quantities of 23 in a comparable yield and selectivity to that of Ni(4-F-PhCN) 4 OTf 2 .…”

“…These results are comparable to what was accomplished with use of the Ni(4-F-PhCN) 4 OTf 2 catalyst. 6b,c Next, we examined the efficacy of Ni(OTf) 2 to mediate stereoselective formation of GlcNα(1→4)GlcA disaccharides 16 and 17 (entries 4 and 5), which are an important subunit of the anticoagulant heparin oligosaccharides 2a and could not be prepared in high yield and α-selectivity with use of in-housed prepared Ni(4-F-PhCN) 4 (OTf) 2 . Accordingly, coupling of armed glucuronic acid acceptor 11 and disarmed acceptor 12 with glucosamine donor 5 to provide α-exclusive disaccharides 16 and 17 , respectively, in moderate to good yield.…”

“…For instance, TMSOTf-mediated coupling with threonine acceptor 6 with the C(2)-azido galactose trichloroacetimidate donor provided the desired product in 55% yield with poor selectivity (α:β = 2:1) even with diethyl ether to serve as putative nucleophile. 22 The threonine-containing compound 24 could be subsequently prepared according to literature 6f or by our improved procedure ( see Scheme 6). 23 The synthesis of compound 24 would allow us to spectroscopically observe its structural variation in different deuterated solvents ( vide infra, Fig.1).…”

“…This was first attempted with the in situ production of HCl from acetyl chloride in methanol, the optimal conditions used for the removal of the benzylidene group of glycosyl threonine amino acid 23 (Scheme 4). 6f Unfortunately, these previous conditions are not suitable for glycosyl serine amino acid 32 because the use of methanol resulted in a transesterification reaction converting an allyl protected acid to a methyl protected carboxylic acid. This methyl ester byproduct came in a 1:1 ratio along with the desired allyl protected intermediate.…”

“…We hypothesize that this is being achieved through a potentially reagent controlled pathway by activation with a substoichiometric amount of nickel. 6 Evolution of this methodology over the years has seen the switch to N -phenyl trifluoroacetimidate donor 3 (both α- and β-anomers now capable of activation) as well as utilizing the inductive effects of the N - ortho -trifluoromethylbenzylidiene group which were able to provide increases in stability and ease of use and preparation of the donors, while maintaining the high α-selectivity of the desired glycoside products 4 (Scheme 1b). …”